This invention relates to a novel electrical resistor structure and process for its manufacture and more specifically relates to a high accuracy, low resistance resistor made with semiconductor device processing techniques.
High current demand equipment such as welders, motors and the like are commonly provided with control circuits which are packaged in compact modules. Such modules commonly use surface mounted semiconductor components. It is necessary for the control function to measure the current drawn by the apparatus being controlled. A very low value current sensing resistor, for example, 1.0 milliohms could be used for this application with such apparatus drawing 10""s of amperes which flow through the current sensing resistor. Conventionally such resistors are comprised of an accurately configured layer of a low resistance material such as well know manganin or aluchrom alloys having a low temperature coefficient of resistance to insure accuracy over a given working temperature range. These devices are commonly separately housed externally of the module and use separate resistor-making technology.
It would be desirable to produce a precise low resistance value, high wattage resistor which can be made in accordance with compatible semiconductor device type technology and could be mounted with power semiconductor die within a module.
A low resistance high current sense resistor, for example, about 1 milliohm and about 10 watt, and having an accuracy of from 1 to 10%, and capable of surface mounting in a high power control module is provided. The device is made using standard semiconductor processing techniques and materials, and can employ fuse links to enable the accurate trimming of the resistor value. More specifically, a standard silicon wafer of the kind used for the manufacture of semiconductor devices such as diodes, MOSFETs, IGBTs, and thyristors and the like is processed, using traditional wafer fabrication techniques, and is diced at the end of the processing to define individual resistor die.
During the processing, the top surface of the wafer is first oxidized to produce a thin silicon dioxide layer. Thereafter, a xe2x80x9cresistivexe2x80x9d layer of any desired material, for example aluminum, titanium or a thermally stable alloy such as manganin is deposited atop the oxide layer. The resistive layer is then photolithographically processed to etch a pattern in the resistive layer to define laterally spaced and distributed resistor regions and electrode regions to define a lateral current path through the resistive layer, having the desired resistance values. The bottom surface of the semiconductor wafer is then metallized with a standard conductive electrode, for example, a traditional chromium, nickel, silver trimetal. In a first embodiment of the invention, the wafer is then diced to define a plurality of identical die suitable for surface mounting on a module heat sink such as a patterned conductive plate, for example an IMS board, along with power semiconductor die.
Conventionally, a low value, high accuracy current sense resistor will have copper terminals which are several hundred microns thick to ensure independence of the final resistor value from the precise location of lead wires on the terminal area. However, copper layers that thick are not compatible with standard semiconductor manufacture. In accordance with a second embodiment of the invention a novel multi-terminal arrangement is provided, employing multiple thin aluminum layers.
Further, in the second embodiment of the invention, and in order to more accurately trim the resistance value of the resistor, the resistive layer may receive a second silicon dioxide layer on the top thereof and a second resistive layer of the same or a different material than the first layer. The second resistive layer is patterned to define a larger number of parallel resistors which make selective, and fusible connections to the underlying resistive layer. The current path along the interconnected first and second resistive layers can then be adjusted by fusing the connection links by applying fusing potentials to selected links.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.